The present invention relates to phase conjugate beams and more particularly to the modulation of phase conjugate beams.
A phenomenon resulting from nonlinear optical mixing is called "phase conjugation" which results from the use of nonlinear techniques for real time processing electromagnetic fields. Phase conjugation is a term which describes a phenomenon involving reversal of the direction of propagation together with a sign reversal of the phase factor. Regardless of the relative orientation of the device and the incident light beam, the reflected conjugate light beam exactly retraces the path of the incident beam. However, there may be a range of particular relative orientations of the device and incident light beams for optimum phase conjugate reflectivity and sensitivity. In addition, a sign reversal of the phase factor (phase reversal) of the incident beam is accomplished. Phase conjugation is sometimes referred to as wavefront reversal.
The properties of the conjugated beam are such that as the wave propagates, the phase of the conjugated beam undergoes a time reversal with respect to the phase properties of the incoming or incident beam. As the conjugate beam propagates away from the nonlinear medium, its phase condition is substantially identical with the phase condition of the incoming beam at the same distance from the nonlinear medium along their respective optical paths. The phase conjugator causes an incoming ray to be "reflected in the phase conjugate sense", to have its propagation vector (or K-vector) inverted such that the incident ray exactly returns upon itself. If a phase aberrator is placed between the source and the phase conjugator, upon the waves retraversal of the aberrator on the way back to the source, the returning wave will have the same beam quality returned at the source as it did when it left the source.
By example and not by limitation, examples of methods for producing optical phase conjugation are four-wave mixing, three-wave mixing, saturated resonances, stimulated Brillouin scattering (or SBS device), stimulated Raman scattering, and photon echo.
In four-wave mixing, also known as degenerate four wave mixing, if al of the beams have the same frequency, the incident incoming beam is mixed with a pair of pump beams and a fourth output beam which is the phase conjugate of the incoming or incident beam, is generated within the nonlinear medium. The two pump beams, emitted either by two external lasers of identical frequency or by one laser with beam splitting optics, produce coherent optical beams which are incident upon a nonlinear medium from opposite directions. An appropriate phase hologram is set up in the medium by the interaction of the two pump waves and an incoming wave incident upon the medium. The incoming wavefront incident upon the pumped medium is reflected as a phase conjugate waveform. Alternately, use of an appropriate absorbing or amplifying medium allows for amplitude holograms to be set up in the medium which results in the phase conjugation process.
In three-wave mixing, commonly known as parametric down-conversion, the incoming wavefront is incident upon the nonlinear medium. Additionally, an external laser emits a pumping beam having twice the frequency component of the incoming beam and is also made incident upon the nonlinear medium from the same direction. The interaction of the waves and the nonlinear medium produce the phase conjugate waveform of the incoming beam to be transmitted by the nonlinear medium, such as a birefringent crystal. This phase conjugate waveform is then transmitted back along the incident optical path by conventional means.
In stimulated Brillouin scattering (SBS), an incident wavefront impinges upon the SBS device and sets up acoustic waves in the SBS medium. The acoustic waves are generated by electostriction wherein there is an interaction of the high intensity electric fields of the laser energy with the SBS medium. The electrostriction process is a spatial periodic modulated response which periodically modulates the density of the SBS medium, setting up acoustic waves in response to the electric field impulses. The acoustic waves that are set up in the SBS medium conform identically to the incident optical wavefronts and act as reflecting surfaces for the incident optical wavefronts which impinge upon these acoustic waves. Thus, the complex phase conjugate image of the incident optical wavefront is reflected. This prcess requires sufficient optical energy due to the existence of a power threshold for the stimulated condition in the SBS device. Any frequency or doppler shift due to the receding acoustic waves in the SBS medium is added to the reflected phase conjugate energy. This shift is usually very small in the order of one part in one hundred thousand and does not affect performance.
The photon echo process is similar to that of the SBS process except that the nonlinear medium is different. In the photon echo process, an incoming incident waveform incident upon the nonlinear medium deforms the medium. The interacting waves are all present at the same time in the medium. A laser pulse emitted by the external laser subsequently incident upon this deformed medium is reflected as the complex phase conjugate waveform of the incident incoming waveform.
It is to be understood that other types of phase conjugate gratings, such as thermal gratings, photo-refractive gratings, etc, are possible and the above is not meant as an all inclusive discussion of the types of phase conjugate gratings and/or wave mixing techniques.
It is often desirable to modulate phase conjugate beams such as for use in communications systems. Usually, this modulation is accomplished by modulating the writing beam such as shown in U.S. Pat. No. 4,496,222 of Shah. However, often there are applications where it is not desirable to modulate the source writing beam(s) in order to modulate the phase conjugate beam.
Accordingly, it is desirable to be able to modulate a phase conjugate beam without modulating the writing beam. Additionally, it is also desirable to modulate the phase conjugate beam by opto-optical means.